I just inaugurated my BD Mercury Mittens high-mountain skiing at Finse, Norway last weekend, and am very pleased with them! First day was cold (-25C) but due to these mittens I hardly noticed. Relatively good breathability although I was toasting inside by the time we reached the peak. Next day was slightly less arctic (-10C) but with high winds. That day I (as usual) donned a lighter pair for the first part of the ascent and switched when the wind-chill set in further up the mountain.
Id say this is a first class mitten in terms of cold and wind protection. Good proportions, good protection around the wrist and easy to use with ski poles. Straps can be adjusted with mittens on. The trigger finger adds to dexterity, but dont expect miracles; I could plot the GPS with the thumb only, and normal zippers can be hard to operate. Opening snowboard bindings etc will however not be a problem. Although I have no intention to take out the inner liners except perhaps for drying, it seems a bit fiddly to put them back in place.
I immediately fell in love with these mittens. I endorse that they must be built for the deepest, coldest days of winter and they will be my pair of choice for the harshest days out skiing and snowboarding. On wormer or wetter days I will however use another pair.